PNEUMOCOCCAL FINDING IN A SAMPLE FROM UPPER AIRWAYS DOES NOT INDICATE PNEUMOCOCCAL INFECTION OF LOWER AIRWAYS

The presence of pneumococcus (Pnc) by antigen detection and culture was examined in nasopharyngeal aspirates (NPA) of 315 children hospitalized with middle or lower respiratory tract infection. Pnc was found in NPA from 115 (37%) patients, being demonstrated by antigen detection alone in 34 (30%), by culture alone in 26 (23%) and by both methods in 55 (48%) of Pnc-positive samples. Pnc findings in NPA were most common, 45-46%, in patients aged 1-4 years. Serological evidence of Pnc infection, based either on detection of Pnc antigen in serum or urine. or on demonstration of an antibody response to these antigens, was present in 31 (27%) of the 115 patients with and in 28 (14%) of the 200 patients without Pnc in NPA samples. In the 48 patients positive for Pnc in NPA samples both by antigen detection and culture the isolated Pnc strains were serotyped. In 45 (94%) of these the type/group of Pnc was the same by both methods indicating that the specificity of the antigen detection tests, latex particle agglutination and counterimmunoelectrophoresis, was high. To evaluate the diagnostic significance of Pnc antigen detection and culture in NPA, sensitivity, specificity and likelihood ratios were calculated; serological evidence of Pnc aetiology was used as a reference. For both methods, sensitivity was poor, < 0.3, but specificity was good, > 0.8. It is concluded that the finding of Pnc by culture or antigen detection in NPA is no indication of Pnc respiratory infection. On the other hand, Pnc etiology is unprobable, if Pnc is not present in NPA. Likelihood ratio of both a positive and negative test result was close to 1.0 indicating that Pnc antigen detection or culture in NPA is not capable to differentiate Pnc infection from other respiratory tract infections.